Levelness refers to an angle between a to-be-measured surface and an absolute horizontal plane. The levelness of the to-be-measured surface can be measured by levelness measuring devices. The levelness measuring devices include bubble levels and electronic level meters. However, the levelness measuring devices need to be in contact with the to-be-measured surface in operation. Thus, it is difficult to measure the levelness of the to-be-measured surfaces that are not accessible or inconvenient to reach using the levelness measuring devices.